I hear the 'fanned frets', whether on a 6 string or a guit with more than 6, are simply more ergonomic. It's just more natural and comfortable with the position of your hands and fingers to have the frets fanned that way. I remember some company touting the ease of playability of frets like that years ago, but for the life of me I can't remember who they were. It's sort of like those damn split ergonomic computer keyboards... supposedly if you get used to them you suffer less wrist and hand fatigue. Of course, I never got used to it and refuse to try again :P
But it makes sense. Think about how your hand and fingers would naturally want to play on the fretboard (further away from you down the neck and more toward you on the higher frets), and how we crank our elbows and wrists to compensate for straight frets.